Hess

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Hess

  1. 1. MODULE 2 AS: Energetics I <ul><li>To understand the principles behind Hess’ Law </li></ul><ul><li>To be able to define the enthalpy of formation and enthalpy of combustion </li></ul><ul><li>To use the definitions to calculate a standard enthalpy change </li></ul>
  2. 2. Exothermic energy level diagram <ul><li>∆ H (‘delta H’) is the symbol for the ‘change in energy’. </li></ul><ul><li>In an exothermic reaction the products have less energy than the reactants. </li></ul><ul><li>∆ H is negative for an exothermic reaction. </li></ul>
  3. 3. Endothermic energy level diagram <ul><li>In an endothermic reaction the products have more energy than the reactants. </li></ul><ul><li>∆ H is positive for an endothermic reaction. </li></ul>
  4. 4. Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.
  5. 5. Definitions <ul><li>Standard enthalpy change of formation is the enthalpy change for the formation of one mole of a compound from its elements under standard conditions </li></ul><ul><li>Standard enthalpy change of combustion is the enthalpy change for the complete combustion of one mole of a substance under standard conditions. </li></ul><ul><li>Standard enthalpy change of neutralisation is the enthalpy change when 1 mole of H+ ions is just neutralised by 1M alkali under standard conditions </li></ul>
  6. 6. Hess’ Law <ul><li>The enthalpy of combustion of pentane is </li></ul><ul><li>-3520 kJ/mol . The enthalpy of formation of carbon dioxide is -395kJ/mol and that of water is -286kJ/mol . </li></ul><ul><li>What is the enthalpy of formation of pentane? </li></ul>

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